An induction heating type of heating apparatus is generally known as a heating section of a hot plate, electric rice-cooker, or the like. In recent years, investigations have been actively pursued into application of this kind of induction heating type of heating section to a fixing apparatus in an image forming apparatus such as a copier, facsimile machine, or printer.
In a fixing apparatus that uses an induction heating type of heating section, magnetic flux generated by a magnetic flux generation section is made to permeate a heat-producing layer of a heat-producing element, and the heat-producing layer is made to produce heat by means of an eddy current generated by the permeation of this magnetic flux. Then an unfixed image formed on recording paper such as copy paper or an OHP (Overhead Projector) sheet is directly or indirectly heat-fixed by heat of the heat-producing element heated by heat production of this heat-producing layer.
Specifically, for example, a heat-producing layer of electrically conductive material is formed on a heat-producing element comprising a fixing roller, fixing belt, or the like. Also, the heat-producing element and a pressure roller on either side of the recording paper feed path are positioned so as to be pressed together, forming a nip that grips and transports recording paper. Furthermore, an exciting coil is wound around a core of ferromagnetic material, forming a magnetic flux generation section, and the exciting coil is positioned opposite the heat-producing layer of the heat-producing element. Then an alternating current of predetermined frequency is applied to the exciting coil, magnetic flux is generated around the exciting coil, forming a magnetic field, and the heat-producing layer of the heat-producing element is made to produce heat by means of an eddy current generated by the action of this magnetic field. In this state, recording paper is transported to the nip between the heat-producing element and pressure roller, and an unfixed image on the recording paper is fixed by heat of the heat-producing element heated by heat production of the heat-producing layer and pressure of the pressure roller.
An advantage of a fixing apparatus that uses this kind of induction heating type of heating section, compared with a heat roller type of fixing apparatus that uses a halogen lamp as a heat source, is that heat production efficiency is higher and the warm-up time required for heating to a predetermined fixing temperature can be shortened.
However, the heating power is great, and so in particular when heating is performed in a low-thermal-capacity fixing apparatus without rotating the heat-producing element comprising a fixing roller, fixing belt, or the like, there is a risk of a localized rise in temperature, and localized thermal destruction of the roller or belt. There is consequently a need for measures such as performing induction heating only during rotation of the fixing roller or fixing belt, and, if heating is performed while the fixing apparatus is in standby mode, rotating the fixing roller or fixing belt at low speed even in standby mode (see Patent Document 1, for example).
If the print mode is changed during continuous printing, the printing speed and the temperature used for fixing may change according to the print mode. For example, if the print mode is switched from plain paper printing to OHP printing, in OHP printing the normal speed is reduced by half in order to maintain permeability, and the temperature used for fixing is also often set higher than the temperature in plain paper print mode. Therefore, when this kind of print mode change is carried out, a phenomenon may occur whereby a fall in the printing speed and a rise in temperature due to a rise in the fixing temperature coincide, and the temperature of the fixing apparatus temporarily exceeds the stipulated value—that is, the phenomenon of overshoot may occur.
With a conventional fixing apparatus that uses a halogen lamp, the above-described change of speed and change of set temperature are performed simultaneously. However, there is a delay in thermal response with a halogen lamp, and heating timing drifts as a result of this thermal response characteristic delay. That is to say, after a change of rotation speed finishes and the temperature of the heat-producing roller has stabilized, a rise in temperature of the heat-producing roller begins. Therefore, overshoot has not been considered to be a particular problem in the case of a conventional fixing apparatus using a halogen lamp.
Patent Document 1: Unexamined Japanese Patent Publication No. 2002-082549